arrow of time

The direction, apparently inviolable, of the "flow" of time
that distinguishes the past from the future. There has been much debate
as to the physical origin of of the arrow of time. Most of physics does
not distinguish between a forward and a reverse direction of time. The one
exception is the second
law of thermodynamics which states that entropy
must increase with time. One possibility is that the overall arrow of time
in the universe is due to its beginning in a state of very low entropy.